To prevent ingredients not to be heated from being overheated by wrapping the whole ingredients with aluminum foil, or the like which reflects microwave in order to prevent the ingredients not to be heated from being irradiated with microwave.

A part with a partition separated from the part to be heated is formed in the lunch box as the part 1 not to be heated. Then, aluminum foil 2 is spread in its bottom part and its side part. A seal obtained by covering aluminum foil 4 with plastic film 5 for cutting off microwave from a remaining upper part is appended to it. Then, heating by a microwave oven is executed after adhering the seal so as to cover and hide the upper part of the part 1 not to be heated by it. On the assumption that the lunch box is heated only in a convenience store, microwave cutting-off metallic sheets covered with the film 5, or the like can be previously distributed to the respective convenience stores in place of appending them and put on the ingredients so as to put the lunch box into the microwave oven. It is preferable that the cap 3 of the part 1 not to be heated is separated from a main body or the partition is also arranged on the cap 3 of the main body by a groove.